One of the great global challenges of the 21st century is that relating to energy: production, distribution and consumption in a more efficient and more environmentally friendly manner. This challenge has become omnipresent in all economic sectors and influences future industrial activity for finding solutions with lower energy consumption.
In this context of reducing energy expenditure, coatings are sought that are able to confine a temperature rise in a cavity, i.e. are able to prevent energy loss to the exterior. For this, the coating must have high reflectivity of thermal radiation (near and medium infrared (IR) range). High reflectivity of thermal radiation means an average reflectivity of infrared radiation in the range 2.5-14.5 μm above 70%.
Generally, in a cavity such as for example a domestic electric oven or a car exhaust system, the walls absorb the infrared radiation emitted by the heating elements. They heat up and in their turn emit radiation extending to the longer wavelengths since their temperature is lower than that of the heating elements. This radiation will partly be absorbed by the objects arranged inside the cavity and the remainder will be absorbed again by the walls. Some of the thermal energy that is not radiated is transmitted by convection within the cavity and by conduction in the various layers constituting the wall, to the external wall of the cavity where it will be dissipated in the surrounding air by convection after raising the temperature of the various layers that it passes through. In order to limit the loss of energy to the exterior of the cavity and improve the energy performance of the equipment in question, one solution consists of trapping the infrared radiation within the cavity, which makes it possible to avoid the energy loss and optionally the recourse to the use of insulating foams. Metallic substrates, optionally metal-coated, are particularly ideal for this because of their high infrared reflectivity. However, there are many drawbacks to their use in this way:
Premature corrosion of the substrate such as in the case of a cold rolled steel (or black iron)
Corrosion in an alkaline medium such as in the case of aluminum or an aluminized coating
Yellowing of the surface during the temperature rise such as in the case of a stainless steel
Absence of cleanability of the surface in contact with dirt
Tarnishing in a damp environment such as in the case of copper or aluminum.
To withstand high temperatures and corrosive and/or oxidizing environments, we generally make use of painted or enamelled surfaces. However, these surfaces have low infrared reflectivity and do not permit energy gains. In contrast, the use of metallic substrates, optionally metal-coated, as cavity walls offers energy gains of the order of 20% relative to painted or enamelled walls.